1. Field of the Invention
The present invention relates to a slot array antenna comprising a plurality of element antennas and including a slot plate having a plurality of slots.
2. Description of the Related Art
An array antenna is of an antenna wherein a plurality of element antennas are disposed in predetermined patterns and having characteristics unobtainable by a single antenna. Controlling the phase of each of the element antennas constituting the array antenna makes it possible to control directivity of the whole array antenna. Accordingly, the array antenna can be utilized even as a beam scan antenna without mechanically activating an array antenna main body.
With remarkable development of wireless communication technologies, frequency bands assigned to various communication apparatuses have been prone to fall short in recent years. In order to make up for it, the effective use of frequencies and the development of technology necessary for shifting to higher frequencies have been imperative. For example, a millimeter wave, which has heretofore been virtually used only for a basic research, has been used in an Intelligent Transport System (ITS). Millimeter wave related communication devices have been expected to be used in car societies like Japan and the U.S and Europe as explosive as home appliances in the near future.
In the field of such millimeter wave communications, it is considered that the use of the millimeter wave becomes essential to various parts and devices. One most-valued device that bears the millimeter wave communications, is of an antenna. The antenna is necessary to transmit and receive a millimeter-wave signal and essential to the millimeter wave communications. Research organizations and makers around the world, which have taken part in the research and development of the millimeter wave communications, are now developing a high-performance millimeter wave antenna in competition with one another. Millimeter wave antennas, which have heretofore been developed, vary widely in terms of configurations. Of these, a slot array antenna is known as a millimeter wave antenna considerably excellent in characteristic.
The slot array antenna is an array antenna wherein as its name implies, slot antennas are disposed in predetermined patterns as element antennas. By determining dimensions and layouts of the respective device slot antennas, desired electric-field distributions can be obtained within predetermined areas. For example, a plurality of slot antennas are two-dimensionally disposed in a square area so that a uniform electric-field distribution related to the direction, phase and amplitude can be obtained. Such an antenna is theoretically nearly identical in radiation characteristic to an apature antenna having a uniform electric-field distribution but is superior to it in terms of the degree of freedom of its configuration and the uniformity of an electric-field distribution.
Disposing the slot array antennas in parallel makes it possible to obtain a widespread and two-dimensional slot array antenna. Such a two-dimensional slot array antenna (hereinafter called simply a “slot array antenna”) has been researched and developed in respective locations. This has been confirmed even experimentally as well as theoretically as one high-gain antenna as will be described in the following reference.
“Prototype Characteristics of Waveguide Slot Array Having 76.5 GHz Low Sidelobe Layer Structure” (Society Conference of the Institute of Electronics, Information and Communication Engineers, March, 2000, B-1-130).
It is common to normally allow a center frequency to have a predetermined bandwidth when it is desired to design a slot array antenna. Namely, if an actually used frequency falls within a predetermined range even if it deviates from a set center frequency, then radiation directivity, an impedance characteristic and a reflection characteristic or the like of the antenna are not degraded. If the frequency falls within a predetermined frequency range with respect to the center frequency, then the frequency can be used. This frequency range (width) is called a “bandwidth”. In general, the more the bandwidth of this frequency is broadened, the more the antenna is evaluated. However, it is not possible to easily broaden the bandwidth.
On the other hand, the bandwidth can be broadened to some extent by allowing a main beam of the radiation pattern of the slot array antenna to have a predetermined angle. The main beam having the radiation directivity of the slot array antenna becomes essentially normal to a slot plate of the slot array antenna. However, if the antenna is designed so that the main beam having the radiation directivity is tilted a few degrees (this angle is called a “tilt angle”) from the direction normal to the slot plate in a slot's longitudinal direction, then the bandwidth can be broadened to a predetermined degree.
However, when the main beam having the radiation directivity has the tilt angle, this will cause inconvenience when the slot array antenna is used for transmission or reception. Namely, while the main beam having the radiation directivity is directed to a target to carry out transmission and reception of an electromagnetic wave, the slot plate does not face in the direction orthogonal to the direction of its transmission/reception. Therefore, a considerable needless space is produced when the antenna is mounted to a transciever. If the direction of the main beam of the antenna is not adjusted, then a lot of trouble is taken over this adjustment.